I actually made a point very similar in one of the many "should the team tear it down or keep on" threads and i'm glad somebody else beat me to it here. Would i call Marchand a shitty leader? Absolutely not. But i do think there's very much something to the fact the room seemingly had all the air sucked out of it with him as C.
Very rarely, if ever is a team led by just one guy in a locker room. As much as fans like to drone on and on about "THAT guy is the heart and soul!" that is never the case. It's usually a group of guys, some wear letters, some don't. Look at past Bruins teams; Chara was obviously the face of the leadership and table setter culture wise, but there were multiple guys around him as well. Savard before he had to retire, Ference, Bergeron, Shawn Thornton, Krejci to an extent. Those guys eventually went on their way but even then, Marchy still had Bergeron alongside him.
Look at the locker room now. Bergy retired and left an enormous hole in the metaphorical heart of the room. Marchand was by himself as the last bastion of that culture with no one to fill it. I love Pasta but he doesn't strike me as the type of guy who much cares for a role like that (and that's okay, that's why you have leaders; so everyone doesn't have to be one.) McAvoy absolutely doesn't strike me as someone smart/firey enough to be one. That's it, that's the core guys.
Who else besides them. Zadorov maybe evolves into that role given he is uber well liked by pretty much every room he's ever been in and plays the right type of game to set a tone.
Edit: This is also why i believe a confrontation between Pasta and Marchy did happen at one point, but not to the stupid degree the media tried to make it out to be. Tensions boiled over and the leadership wasn't there like it used to be.
